The present invention relates to a gripper system and method for a cylindrical sheet guide and, more particularly but not exclusively, to a sheet-fed rotary printing press, as well as to a sheet-processing machine having a gripper system and a method for use thereof.
In most printing presses, sheet-fed rotary printing mechanisms are used. Each sheet-fed rotary printing mechanism integrates one or more cylindrical sheet guides in order to process the sheets and pass them among the different assemblies of a printing press. For example, most high-speed, sheet-fed rotary printing presses are provided with an impression drum, which is used for printing. Cylinders which permit the press to print on two sides of a sheet, and which can also be used for single-sided, multi-color printing are also commonly used.
In order to retain sheets on cylindrical sheet guides and in order to allow the transferring of sheets to and from the cylindrical sheet guides, grippers are usually used. The grippers are needed in order to facilitate the loading and unloading of sheets to and from the cylindrical sheet guides. Transmission elements and adjusting mechanisms are usually needed in addition to the grippers in order to coordinate among the printing press assemblies during the loading and unloading of the sheets and in order to allow different movement patterns of the sheets within the printing press. A turning impression drum, for example, is usually designed with spaces to receive grippers and transmission elements and has a cylindrical sheet support surface operative to guide the sheets, which extends over more than half the surface of the turning impression drum. Assembly and maintenance of these systems are, therefore, often fairly complex.
German Patent No. 4200406, published on Jul. 15, 1993, discloses a gripping device that integrates sensors. The device includes grippers for a sheet-like material which are fastened to a gripper-operating shaft. The grippers are simultaneously opened and closed via a cam control device. Individual grippers or groups of grippers on the edge of a cylindrical sheet guide may be activated independently of one another by a control device. The control device includes at least one sensor, an opening and closing mechanism for the grippers and a computer. The sensors are angle-of-rotation sensors or sheet-position sensors, which detect either the position of the cylinder or the position of the sheet-like material, in order to actuate the opening and closing mechanism.
The device has a mechanical cam mechanism. Such mechanisms are disadvantageous in that, usually, a fixed mechanical coupling is provided for the main machine drive and, due to its dimensioning, it is not possible to change the movement thereof. Therefore, movement of the machine drive cannot readily be adapted during operation to changing boundary conditions, e.g., a change in the thickness of printing-materials. Moreover, since the system is subjected to disruptive forces during the gripper actuation, this causes the machine to be subjected to a retroactive torque effect.
In order to avoid the aforementioned disadvantages and because the movement patterns are so complex, the grippers must be carefully designed and controlled. Since, today, mechanical processes are computerized and are hardware and software based, it is preferable that control of the grippers be done by integrating such hardware- and software-based processes into gripper control mechanisms.
For example, U.S. Pat. No. 6,877,430, issued on Apr. 12, 2005, discloses a gripping device for a sheet processing machine that comprises grippers that may be adjusted during the printing process. The gripping device includes a gripper, a gripper pad associated with the gripper and a first drive for moving the gripper from an open position to a closed position. The gripper is engageable with the gripper pad for producing a clamping force for holding sheets being processed. A second drive is operable independent of the first drive, during a printing operation of the sheet-processing machine. The first and second drives position the gripper and the gripper pad so that clamping faces of one are moved so that they are perpendicular to clamping faces of the other.
In one embodiment, the second drive comprises a piezoelectric sensor that measures a compressive force in order to calculate the desired position values for the gripper pad. These values are fed to the power electronics in order to move the second drive into the appropriate position until the actual value of the clamping force, which is determined via the force sensor, corresponds to the desired value for the clamping force determined by the control device.
Since the gripper pads of the device are not vertically adjustable, the device does not have to be adapted for different sheet thicknesses so as to avoid deformations of the sheet in the region of the grippers. The device thus overcomes some of the disadvantages of other such devices. However, there is no means for allowing the printing press to optimally adjust the opening and closing of the grippers, depending on the sheet properties, during the printing process. There is thus a widely recognized need for, and it would be highly advantageous to have, a gripper system devoid of the above limitations.